1. Field of the Invention
The present invention relates to a process for producing 1,3-butadiene (hereinafter, simply referred to as "butadiene"), and more particularly to a process for producing butadiene from a C.sub.4 -hydrocarbon fraction.
2. Description of Prior Art
The production of butadiene by the dehydrogenation or oxidative dehydrogenation of 1-butene and/or 2-butene (hereinafter, sometimes referred to as "n-butuenes") in the presence of a selective catalyst is well-known, and is practised on an industrial scale. The n-butenes used as the starting material in this process are obtained by, for example, the dehydrogenation of n-butane, and also formed as by-products in the fluidized catalytic cracking (FCC) of heavy oil or the steam cracking of naphtha, kerosene, light oil or the like.
Amongst these C.sub.4 -fractions, by-products in the steam-cracking of naphtha or the like contain a large quantity (for example, 25-50% by weight) of butadiene. Therefore, the butadiene is first separated from the by-products by extraction, the residual fraction (spent C.sub.4 fraction) is used as a source for n-butenes.
The C.sub.4 -fraction obtained as a by-product in the FCC and the spent C.sub.4 -fraction obtained in the steam-cracking are mixtures comprising i-butane, i-butene, n-butane and the like in addition to n-butenes. It is preferred that the starting material used in the production of butadiene by dehydrogenation or oxidative dehydrogenation satisfies the requirements that the n-butenes concentration in C.sub.4 -fraction is high and substantially no i-butene is contained. For example, UOP, 1978, Technology Conference-H-11 describes that the n-butenes concentration in the starting material for the production of butadiene may be about 90% or more but the i-butene concentration in the starting material must be about 0.3% or less.
Separation of a specific component from a C.sub.4 -fraction by the conventional distillation is practically impossible, because the boiling points of components contained in the C.sub.4 -fraction are so close to one another as shown in Table 1, and a variety of methods for the separation have hitherto been studied and developed.
TABLE 1 ______________________________________ Main C.sub.4 hydrocarbons Boiling points (.degree.C.) ______________________________________ i-Butane -11.7 i-Butene -6.9 1-Butene -6.3 1,3-Butadiene -4.4 n-Butane -0.5 t-2-Butene 0.9 c-2-Butene 3.7 ______________________________________
Table 2 shows conceptionally how C.sub.4 -paraffins and C.sub.4 -olefins are separated by main methods for separating C.sub.4 -hydrocarbons.
TABLE 2 ______________________________________ Separation method Component A B C D E F G H ______________________________________ i-Butane x x .circleincircle. .circleincircle. x x .circleincircle. .circleincircle.-n-Butane o x .circleincircle. .circ leincircle. o x .circleinc ircle. .circleincircle. i-Butene x .circleincircle. x x x .circleincircle. .circleincircle. x 1-Butene x .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. x .circleincircle. t-2-Butene .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. c-2-Butene .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. ______________________________________ Note: (1) Separation methods A: Distillation B: Extractive distillation C: Absorption by sulfuric acid (separation of ibutene by absorbing it in 50-65% by weight sulfuric acid) D: Etherification (separation of ibutene by converting it to methyl tbuty ether or the like) E: Vapor phase adsorption (Olefin SIV (registered trade mark) method UCC F: Liquid phase adsorption (C.sub.4 OLEX (registered trade mark) method UOP) G: Liquid phase adsorption (SORBUTENE (registered trade mark) method UOP H: Dimerizing distillation (separation of ibutene by dimerizing it) (2) Degree of separation from 2butene .circleincircle.: Mostly entrained o: Considerably entrained x: Mostly separated
As is apparent from Table 2, there is no single method by which the n-butenes for use as a starting material for the production of butadiene by dehydrogenation or oxidative dehydrogenation, which n-butenes contain substantially no i-butene and have been substantially freed from C.sub.4 -paraffins, can be separated from the above-mentioned various, industrially available C.sub.4 -fractions in a high yield, and such a separation requires combining two or more methods. For example, Sekiyukagaku Kogyo Handbook (Asakura Shoten, 1962), p. 178, FIGS. 7 and 6(B), illustrates a flow sheet for treating a C.sub.4 -fraction first in sulfuric acid-absorption apparatus to remove i-butene therefrom and then subjecting the residue to extractive distillation to obtain n-butenes. Further, Oil & Gas Journal, 55 (48), p. 87 states a process which comprises feeding the starting C.sub.4 -fraction to a starting material-preparing step comprising a sulfuric acid absorption unit and an extractive distillation unit to obtain a n-butene fraction freed from i-butene, i-butane and n-butane, feeding the said n-butene fraction to a butadiene synthesis step comprising a dehydrogenation unit and a product gas-treating unit to obtain a crude butadiene fraction, feeding the said crude butadiene fraction to a butadiene-purification step to obtain purified butadiene and at the same time to separate and recover the unreacted C.sub.4 -fraction composed mainly of n-butenes, and recycling the unreacted C.sub.4 -fraction thus recovered to the butadiene-synthesis step and/or the starting material preparation step. Further, UOP, 1978, Technology Conference H-23 describes a process for treating the starting C.sub.4 -fraction first in a sulfuric acid-absorption unit to remove i-butene therefrom and then feeding the residual fraction to a C.sub.4 -OLEX unit to remove butanes and to obtain n-butenes.
In all the processes mentioned above, the sulfuric acid absorption method is used for removing i-butene. As is apparent from Table 2, the use of etherification, vapor phase adsorption or dimerization in place of the sulfuric acid-absorption method can easily be predicted.
In the accompanying drawings, FIG. 1 shows the whole process mentioned in Oil & Gas Journal, 55 (48), p. 87 which is a combination of the above-mentioned step of producing n-butenes (the starting material preparation step), the step of synthesizing butadiene by dehydrogenation or oxidative dehydrogenation and the step of purifying butadiene to obtain highly purified butadiene. That is, the starting C.sub.4 -fraction comprising C.sub.4 -paraffins and C.sub.4 -olefins is first sent to the starting material-preparation step where i-butene and C.sub.4 -paraffins are substantially completely removed thereform. The n-butene fraction thus obtained is sent to the butadiene-synthesis step, and the resulting crude butadiene fraction is sent to the butadienepurification step. In the butadiene-purification step, the unreacted C.sub.4 fraction is separated from butadiene and the major part of the unreacted C.sub.4 -fraction is returned to the butadiene-synthesis step. If separation of C.sub.4 -paraffins in the starting material-preparation step is insufficient, the C.sub.4 -paraffins are gradually accumulated in the system to seriously disturb the operation because C.sub.4 -paraffins are inert under the conditions of usual catalytic dehydrogenation or oxidative dehydrogenation of n-butene. For this reason, a part or the whole of the recycle stream from the butadiene-purification step is returned to the starting material-preparation step.
However, the production of n-butenes by the combination of the above-mentioned existing techniques becomes not only complicated in steps but also high in running cost. In addition, considering the competition of the butadiene produced from the n-butenes with the butadiene in the C.sub.4 -fraction formed as a by-product by the steam cracking of naphtha or the like, the former butadiene has a serious economical problem and is actually required to be improved.
The present inventors have conducted studies with the aim of solving the problems which the existing techniques have. As a result, they have reached an idea that a more rational process may be constructed by considering the starting material-preparation step for producing butadiene by dehydrogenation or oxidative dehydrogenation not independently but along with the butadiene-purification step, the two steps having a common problem of separating each of the components contained in the C.sub.4 -fraction. After an elaborate study for realizing the above idea, it has been found that utilizing the fact that n-butane which is difficult to separate together with i-butene from n-butenes in the starting material-purification step can easily be separated from n-butenes according to the extractive distillation method widely used in the butadiene-purification step, and it is allowable for butanes to be incorporated to a certain extent into the n-butenes to be fed to the butediene-synthesis step, the C.sub.4 -fraction freed substantially from i-butene (when lighter and/or heavier fractions than C.sub.4 are contained, they have been removed by distillation or the like) can be fed to the middle stage of the extractive distillation column after or without passing through the butadiene-synthesis step, and a fraction composed mainly of C.sub.4 -paraffins can be distilled out of the column top while withdrawing a side vapor stream from a stage on the upper side of the stage of feeding the C.sub.4 -fraction containing butadiene from the butadiene-synthesis step and on the lower side of the stage of feeding a selective solvent that is near tne top of the column, whereby a fraction rich in n-butenes and containing decreased concentrations of C.sub.4 -paraffins and butadiene can be obtained as the side vapor and, on the other hand, a fraction consisting of the selective solvent and hydrocarbons composed mainly of butadiene and containing a small quantity of 2-butene, can be obtained from the bottom of the column.